1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a plastic-encapsulated semiconductor device having a good heat dissipation performance and a good moisture resistance, and a fabrication method thereof.
2. Description of the Prior Art
Conventionally, plastic-encapsulated semiconductor devices have been used in various electronic equipment. In the semiconductor devices of this sort, typically, an Integrated circuit (IC) or Large-Scale Integrated circuit (LSI) chip or pellet is encapsulated in a plastic package and electrically-conductive leads or terminals are protruded from the package. These leads are electrically connected to boding pads of the IC chips within the package. If the semiconductor device is mounted on a circuit board and the leads or terminals are electrically connected to wiring lines of the circuit board, various input signals may be applied to the IC chip or pellet and various output signals may be derived therefrom.
The semiconductor devices of this sort have been practically used in various application fields. With the semiconductor devices designed for outputting the electric wave in portable or pocket telephones, electric power consumption generates a lot of heat on operation and therefore, the heat thus generated needs to be efficiently dissipated or radiated.
FIGS. 1 and 2 show a conventional plastic-encapsulated semiconductor device 501 capable of efficient heat dissipation. This device is disclosed in the Japanese Non-Examined Patent Publication No. 61-144834 published in 1986.
As shown in FIGS. 1 and 2, this plastic-encapsulated semiconductor device 501 is equipped with an approximately square IC chip or pellet 503 having bonding pads 502 on its top face. The chip 503 is mounted on an island 504. The island 504 is formed by a rectangular metallic plate and the chip 503 is fixed onto the center of an upper side of the island 504. Leads or terminals 505 are arranged at regular intervals on the long sides of the island 504. The leads 505 are electrically connected to the corresponding bonding pads 502 of the chip 503 through bonding wires 506.
The IC chip or pellet 503, the island 504, the leads 505, and the bonding wires 506 are encapsulated or molded in a rectangular parallelepiped-shaped plastic package 507 while the outer parts of the leads 505 and the short-side ends of the island 504 are protruded from the package 507. The protruding outer parts of the leads 505 are located at the long sides of the package 507. The protruding ends of the island 504 have penetrating circular holes 508.
The conventional semiconductor device shown in FIGS. 1 and 2 is mounted, for example, on an upper side of a wiring board (not shown) with the use of screws inserted into the penetrating holes 508 of the island 504. The leads or terminals 505 are electrically connected to wiring lines formed on the upper side of the wiring board. Various electric signals are transferred between the chip 503 and the wiring lines through the terminals 505. The heat generated in the chip 503 is dissipated at the protruding ends of the island 504 from the package 507.
With the conventional semiconductor device 501 shown in FIGS. 1 and 2, the heat generated in the IC chip 503 is dissipated through the island 504. However, the protruding parts of the island 4 from the package 507 are apart from the bottom of the package 507. Therefore, conventionally, to improve the heat dissipation performance or capability, a proper thermally-conductive spacer is sandwiched between the protruding ends of the island 504 and the circuit board, or a proper heat-releasing plate is fixed onto the top of the package 507. These additional members increase the number of parts and fabrication processes, thereby lowering the productivity and upsizing the semiconductor device 501.
Also, since the IC chip or pellet 503 is located at the middle of the elongated island 504, the protruding short-side ends of the island 504 are apart from the chip 503 at a comparatively long distance. Accordingly, the conventional semiconductor device 501 has no satisfactory heat-dissipation performance.
Further, although the semiconductor devices of this sort designed for the portable telephones have been required to be mounted on a wiring board as flat as possible in view of downsizing, the conventional semiconductor device 501 is not as flat as required.
FIGS. 3 and 4 show another conventional plastic-encapsulated semiconductor device 521, which solves the above-described problems in the conventional plastic-encapsulated semiconductor device 501. This device is disclosed in the Japanese Non-Examined Patent Publication No. 2-63142 published in 1990.
The explanation about the same configuration is omitted here by attaching the same reference numerals as those in FIGS. 1 and 2 to the same or corresponding elements in FIGS. 3 and 4 for the sake of simplification.
As shown in FIGS. 3 and 4, this plastic-encapsulated semiconductor device 521 is equipped with a heat-releasing plate 523 with a circular-ringed shape. The heat-releasing plate 523 is protruded from one side of a plastic package 522 from which the leads or terminals 505 are not protruded. The heat-releasing plate 523 is formed to be incorporated with an island 524.
The heat-releasing plate 523 is located on a same plane as a middle part 525 of the island 524 on which the IC chip 503 is mounted. As clearly shown in FIG. 4, the middle part 525 of the island 524 is exposed from the bottom face of the package 522. The inner parts of the leads or terminals 505, which are arranged at each side of the island 524, are also exposed from the bottom face of the package 522.
Therefore, the conventional semiconductor device 521 is able to be mounted as flat as required for the portable telephones.
The heat-releasing plate 523 has a circular penetrating hole 526. The conventional semiconductor device 521 is mounted on a wiring board (not shown) with the use of a screw inserted into the hole 526.
With the conventional semiconductor device 521 shown in FIGS. 3 and 4, since the leads or terminals 505 are exposed from the bottom face of the package 522, the leads 505 can be directly fixed onto the wiring lines of a wiring board by solder joints. The semiconductor device 521 is mounted on a wiring board by fixing the heat-releasing plate 523 protruding from the package 522 onto the wiring board with the use of a screw and fixing the island 524 onto a grounding wiring line of the wiring board.
If the middle part 525 of the island 524 is located on an electrically conductive pattern of the wiring board, the middle part 525 can be connected to the pattern by an adhesive with a good thermal conductivity. In this case, the heat generated in the IC chip 503 is efficiently transmitted to the pattern of the wiring board through the heat-releasing plate 523 and the middle part 525 of the island 524. Thus, the heat-generating chip 503 is efficiently cooled.
However, the conventional semiconductor device 521 shown in FIGS. 3 and 4 has the following problems.
Specifically, in the conventional semiconductor device 521, not only the leads 505 but also the middle part 525 of the island 524 are exposed from the bottom face of the plastic package 527 in order to improve the heat dissipation capability. However, the heat dissipation capability is not practically improved unless the middle part 525 of the island 524 is connected to the wiring board by an adhesive with a good thermal conductivity. In this case, the thermally-conductive adhesive tends to flow toward the leads or terminals 525 before curing and therefore, contact or connection failure of the terminals 525 will occur.
To prevent the contact or connection failure of the terminals 525, the terminals 525 may be connected to the wiring lines of the wiring board by solder joints in a soldering process of the terminals 525, instead of the thermally conductive adhesive. In this case, however, a solder is difficult to flow into the narrow gap between the bottom face of the package 522 and the upper surface of the wiring board. Thus, even if the middle part 525 of the island 524 is exposed from the package 522, heat dissipation through the middle part 525 of the island 524 is not practically expected.
The exposure of the middle part 525 of the island 524 increases the possibility that the moisture contained in the atmosphere reaches the IC chip 503 through the exposed middle part 525. The moisture will facilitate (a) disconnection or release of the bonding wires 506, (b) electrical short-circuit between the terminals or leads 505 due to the migration phenomenon, and (c) time-dependent degradation of the chip 503, thereby decreasing the lifetime of the semiconductor device 521.
From this view point, the possibility that the moisture contained in the atmosphere reaches the chip 503 is low in the conventional semiconductor device 501 shown in FIGS. 1 and 2, because only the short-side ends of the island 504 are exposed from the package 507. In this case, however, as already described above, there arises a problem that a proper spacer or heat-releasing plate is necessary and direct connection of the island 504 to the wiring board is difficult.
Moreover, with the conventional semiconductor devices 501 and 521, since the ends of the islands 504 and 524 are protruded from the plastic packages 507 and 522, respectively, they are upsized. From the view point of application to the portable telephones, the upsizing of the devices 501 and 521 will make an important issue.
Additionally, when high-frequency electric signals are used, the high-frequency electric signals are transferred through a part of the leads or terminals SOS. In this case, interference tends to occur between the leads 505 for the high-frequency electric signals and the remaining leads 505 adjoining thereto. To cope with the interference, it is preferred that grounded terminals or leads are additionally provided at each side of the leads 505 for the high-frequency electric signals. However, no consideration exists in the conventional semiconductor devices 501 and 521.